Injection of unprepared embryonic stem cells into the body sounds like a good way to generate cancers rather than regeneration, but why not try it in animal models? From Betterhumans: "Injecting embryo stem cells 'straight up' - without changing their cell type first - has fixed damage in an animal model of heart attack. ... the stem cells, when transplanted into damaged mouse hearts, morph into functional forms of cells that compose a healthy heart. The study is considered important because it means that blank-slate embryonic stem cells could be introduced directly to damaged heart tissue to repair heart muscle and blood vessels. ... One intriguing result of the new study is that the implanted cells did not result in tumor formation, one of the primary safety concerns for stem cell therapy."